Pipes or ducts that convey a fluid at a temperature that differs from the temperature of the ambient medium are generally provided with lagging that surrounds them and that performs two or three functions, namely: providing thermal insulation, protecting said pipes or ducts, and also protecting personnel who may be called upon to move about in the vicinity of said pipes or ducts when they are located at ground or floor level or within reach of a person.
Proposals have thus been made to provide such ducts or pipes with lagging made of foamed plastics material, e.g. PVC, and although such lagging is satisfactory with respect to providing thermal insulation, it is nevertheless mechanically fragile, and is also unsatisfactory for implementation in confined spaces (mine galleries, submarines, etc . . . ) where it is required that the lagging used should present a certain amount of resistance to fire and that it should not give off toxic smoke or vapor in the event of a fire. Consequently, in such applications, conventional lagging made of plastics foam is unsuitable, and instead use is made of casing based on mats of resin-impregnated fiberglass of considerable thickness, the mats being built up and allowed to set in situ, or in a variant being applied to lengths of pipe in workshops fitted for that task. Although the results obtained are satisfactory, this is achieved only at the cost of difficulty in implementation when the pipes are covered in situ, or else at the cost of a very large amount of work to dismantle the pipes, to transport them from the locations at which they are installed to lagging factories, and back again, etc . . . such that when it is necessary to equip a building or a ship that has a very large number of pipes or ducts of complex shape, then installing such insulating lagging gives rise to considerable constraints.